UV unwrap: separate maps for separate faces problem
Yes you can have separate textures for separate faces. 

One technique is to scale the vertices of unwanted faces by zero to the median point. Scale 0.  The area of the faces are now zero. Zero area signals no mapping is attempted.  I hope the single dot on the uv map is acceptable to you.

 


mark seams so as to partition faces into group A and group B


First UV Slot


uv unwrap case 1 and move unwanted faces to the side of uv map. The texture repeat must be set to [clip].


Second UV slot


uv unwrap case 2 and move unwanted faces to the side of uv map. The texture repeat must be set to [clip].
The combination of faces of the uv map and repeat set to [clip] should achieve your goal


Keep in mind advanced users may be able to simply use one uv map with texture painting.  You are the advanced user.